In order to reduce effects due to a nonzero impedance, it is of interest to minimize the impedance. Further, in order to achieve mathematically complete, and therefore ideal, load invariant operation, it can be shown that an impedance of interest should be forced to zero. All known techniques produce less or more successful minimization of the impedance of interest, as a rule in a proportion to their complexity. None of the presently known techniques produces a zero impedance, except a synthesis method described in a copending and coassigned application by these same two inventors Lj. Dj. Varga and N. A. Losic, entitled "Synthesis of Zero-Impedance Converter", filed December 1989. A specific and particular applications of a zero-impedance converter, in addition to those in the application above, are described in the U.S. Pat. No. 4,885,674, entitled "Synthesis of Load-Independent Switch-Mode Power Converters" by Lj. Dj.Varga and N. A. Losic, issued December 1989, as well as in a two copending and coassigned applications of N. A. Losic and Lj. Dj. Varga, entitled "Synthesis of Load-Independent DC Drive System", U.S. patent application Ser. No. 07/323,630, November 1988, and "Synthesis of Load-Independent AC Drive Systems", U.S. patent application Ser. No. 07/316,664, February 1989 (allowed for issuance December 1989).
In all these applications, the zero-impedance converter, as a potentially unstable system, is made stable by necessarily closing a negative feedback loop(s) around the zero-impedance converter, those loops belonging to a system to which the zero-impedance converter is being applied. For example, in applying a zero-impedance converter to synthesizing load-independent switch-mode power converters an outer negative voltage feedback loop is closed around the zero-impedance converter by sampling the switch-mode power converter output voltage and feeding it back in the negative feedback loop. In applying a zero-impedance converter to synthesizing load-independent electric motor drive systems, a negative velocity and position feedback loops are closed around the zero-impedance converter by employing appropriate sensing devices, e.g. tach and encoder.
While closing the outer negative voltage feedback loop in case of a switched power converters incorporating zero-impedance converter is necessary, i.e., it is not redundant, the closing of negative velocity and position loops, in case of electric motor drives incorporating zero-impedance converter, can be avoided by improvement of the zero-impedance converter. Furthermore, and as suggested in a copending and coassigned application by N. A. Losic and Lj. Dj. Varga, entitled "Synthesis of Drive Systems of Infinite Disturbance Rejection Ratio and Zero-Dynamics/Instantaneous Response", U.S. patent application Ser. No. 07/468,122, January 1990 , for case of a zero-impedance converter, the improved zero-impedance converter may be used to provide for a synthesis of drive systems of the same properties as above but without the need for closing position and velocity loops.